1. Field of the Invention
This invention relates to a gasification-type combined electric power generating plant which generates electricity using a gasifying furnace supplied with fuel (such as coal or heavy oil), a gas turbine, and a steam turbine.
2. Description of the Prior Art
FIG. 4 shows a conventional gasification-type combined electric power generating plant which uses coal (pulverized coal) as fuel.
In this drawing, the coal-gasification-type combined electric power generating plant comprises a gasifying furnace 1 for performing partial oxidation under high-temperature and high-pressure conditions, a heat exchanger 2 for recovering the sensible heat of a high-temperature fuel gas generated, a gas purifying device 3 for eliminating minor pollutive components and dust out of the fuel gas generated, a gas turbine 4 driven upon combustion of the purified fuel gas, an exhaust heat recovering boiler 5 for recovering the heat of the high-temperature gas at the outlet of the gas turbine, and a steam turbine 6 driven by the steam generated by the recovered heat.
Air (inclusive of a gasifying agent, oxygen-containing gas, etc.) to be supplied to the gasifying furnace 1 is bled from a gas turbine compressor 4a, and boosted by a secondary air booster 9 to predetermined pressure for a primary air 14 and secondary air 13 used in the gasifying furnace 1. Before boosting, the gasifying agent is cooled in an air cooler 32 by low-pressure feed water 17 or the like for the purpose of increasing the efficiency of compression, and after boosting, the secondary air 13 is heated in an air heater 31 by bleed air 33 from the steam turbine 6, and then supplied to the gasifying furnace 1. At the time of startup, the primary air and the secondary air are boosted by a starting compressor 55 to a predetermined high pressure for the primary air.
In FIG. 4, 4b is an output section of the gas turbine 4, 8 an air cooler, 9 the secondary air booster (which also boosts the primary air), 11 a primary air cooler, 12 a coal supply device, 14 the primary air, 15 a condenser, 16 a condensate pump, 18 a feed water pump, 19 a high-pressure feed water, 20 the main steam of the gasifying furnace, 21 high-pressure main steam, 22 reheat steam, 23 low-pressure steam, 24 a chimney, 56 an air heater, and 57 an air cooler.
For convenience of illustration, the primary air 14, for example, represents the primary air itself and the supply passage thereof. That is, the passage of the primary air is not illustrated in a separate form. This manner of illustration is applied to the bleed air 33 and the like, and to embodiments described later.
The conventional gasification-type combined electric power plant has the following problems to be solved:
(a) Since the bleed temperature of the gas turbine compressor is high (on the order of 300.degree. to 400.degree. C.), even when heat recovery is performed by cooling with a low-temperature feed water of low pressure (on the order of 30.degree. to 60.degree. C.), the temperature drop is large and the energy decrease is significant, leading to poor heat recovery.
(b) Since the air (inclusive of gasifying agent, oxygen-containing gas, etc.) after boosting is heated by the turbine bleed air of high energy although the heat-gas efficiency of the gasifying furnace and the power generation efficiency of the gas turbine are increased a little, the bottom cycle efficiency of the water-steam system is decreased.
(c) Since only one booster is used to boost the primary and secondary air, the pressure loss arising in controlling the air to each predetermined pressure is large, demanding an increase in necessary power.
(d) Since the starting compressor is switched to handle either a rated load or a large load, its power and pump head must be compatible with the predetermined pressure, flow rate and temperature of the gasifying furnace under rated or large load conditions, resulting in size enlargement.
(e) To attain a predetermined temperature demanded by the gasifying furnace, there must be provided an air cooler and an air heater.
(f) To replace the starting compressor after the starting of the gas turbine, there must be provided a blow-off switching device of large size and high differential pressure.
In this connection, Japanese Patent Laid-Open No. 62-186018 discloses a system wherein compressed air bled from a gas turbine compressor is subjected, in a regenerative heat exchanger, to heat exchange with another compressed air from the outlet of a booster and then supplied to a gasifying furnace. In such a gasifying furnace for partially oxidizing a carbon-containing fuel (such as coal or heavy oil), however, an oxygen-containing gas such as the compressed air serves as a gasifying agent and functions (in the form of primary air) to convey coal into the gasifying furnace, or spray heavy oil into the gasifying furnace. Therefore, the booster must have a high pressure ratio because a large pressure loss is caused by coal conveying and heavy oil spraying.
Accordingly, in the prior system wherein the oxygen-containing gas (an air-like gas) being supplied to the gasifying furnace is boosted by one booster, it is necessary to boost the whole air, inclusive of the secondary air, to a maximum pressure required to operate the gasifying furnace, i.e. the pressure of the primary air.
Further, a wet-type slurry supply system, known as one coal supply system is, inferior to a dry-type supply system, which is small in moisture evaporation and in latent heat loss. A heavy oil supply system using a steam spray is inferior in thermal efficiency to an air spray system (which is small in energy). Such known systems are defective in that the pressure loss of a coal or heavy oil supply line is large, and thus the plant-home power of the booster must be increased.
Generally, as the primary air, about 25% of air supplied to the gasifying furnace is used.